1. Field of the Invention
This invention relates to an electrolytic erosion preventing insulated rolling bearing and a manufacturing method thereof. This electrolytic erosion preventing insulated rolling bearing is incorporated in a rotation support portion into which there is a possibility of current flowing, such as a rotation shaft of a general-purpose electric motor for general industry, an electrical generator (electrical generator of a wind mill or the like), a main motor for a rail car, or a medical device (CT scanner or the like). Specifically, this invention relates to a large sized electrolytic erosion preventing insulated rolling bearing whose outer diameter is 200 mm or more.
Moreover, this invention is a bearing assembly which constitutes a rotation support portion of a general-purpose electric motor for general industry, an electrical generator (electrical generator of a wind mill or the like), a main motor for a rail car, or a medical device (CT scanner or the like). Specifically, this invention relates to a bearing assembly which is incorporated in an inverter controlled motor or generator.
2. Description of Related Art
In the case of a rolling bearing for supporting a rotating shaft as used in a range of electrical equipment such as an electric motor, an electrical generator, or the like, if no countermeasure is devised, electrical current, such as return current, motor shaft current, or the like, flows into the rolling bearing itself. In the case where electrical current flows into a rolling bearing, so-called electrolytic erosion occurs whereby erosion progresses in the parts that serve as pathways of current, shortening the life of the rolling bearing significantly. Conventionally, in order to prevent such electrolytic erosion from occurring, an electrolytic erosion preventing insulated rolling bearing has been known, that prevents electric current from flowing into the rolling bearing by the formation of an insulating coating on the surfaces of the outer ring and the inner ring of the rolling bearing, as is disclosed in Patent Documents 1 to 3, and Patent Documents 8 to 10, for example.
The insulated rolling bearings disclosed in each of the patent documents are produced by forming an insulating coating such as a ceramic, a synthetic resin, or the like, on part of a bearing ring of the rolling bearing that fits with and is supported by a mating component, and is configured as shown in FIG. 34 for example. The rolling bearing is provided with a plurality of rolling elements 5 between an inner ring raceway 2 formed in the outer peripheral surface of an inner ring 1 and an outer ring raceway 4 formed in the inner peripheral surface of an outer ring, that enable the inner ring 1 and the outer ring 3 to rotate relative to each other. An insulating coating 6, which is a ceramic sprayed layer, is formed on the outer peripheral surface and the two axial end faces of the outer ring 3. In the case of such an electrolytic erosion preventing insulated rolling bearing, in a state in which the outer ring 3 is fitted inside and supported by a metal housing, the insulating coating 6 insulates the outer ring 3 from the housing. As a result, current does not flow between the outer ring 3 and the housing, preventing electrolytic erosion as described above from occurring in each of the components 1, 3 and 5 of the rolling bearing.
However, in the case of the known conventional electrolytic erosion preventing insulated rolling bearing, which is disclosed in the above-mentioned Patent Documents 1 to 3, it is difficult to ensure high levels of insulation performance, durability, and low cost, at the same time. The reason is as follows. For example, in the case where the insulating coating 6, which is a ceramic sprayed layer, is formed on the surface of the outer ring 3 by moving a spray nozzle along the outer peripheral surface 7 of the outer ring 3, a ceramic sprayed layer is formed on the outer peripheral surface 7, and by moving a spray nozzle along the two axial end faces 8 of the outer ring 3, ceramic sprayed layers are formed on the two axial end faces 8. As a ceramic sprayed layer formed in this manner, one with a thickness dimension of 0.5 mm or more (typically about 0.6 to 0.7 mm) is formed conventionally by spraying droplets of ceramic material containing alumina (Al2O3) at 94 to 95 percent by weight.
Since ceramic sprayed layers are formed on the surfaces 7 and 8 as described above, ceramic sprayed from both nozzles becomes attached to continuous folded over portions 9 which are located between the two axial end faces 8 and the outer peripheral surface 7. Consequently, the thickness dimension of the continuous folded over portions 9 is greater than the thickness dimension of the outer peripheral surface and the two axial end faces 8. As a result, if the thickness dimension of the surfaces 7 and 8 is made a sufficient value from the aspect of maintaining insulation performance, the thickness dimension of the two continuous folded over portions 9 becomes too large. The ceramic sprayed layers are brittle, so if the thickness dimension becomes too large, it is likely to cause damage such as cracks, chips and the like. Since the continuous folded over portions 9 themselves do not make contact with other parts such as the housing and the like, even if the ceramic sprayed layer is damaged, it is unlikely to cause a problem from the aspect of maintaining the insulation performance. However, in the case where a broken fragment of ceramic falls and enters the inside of the rolling bearing, it is likely to generate damage such as making an indentation in the surface or the like of the inner ring raceway 2, the outer ring raceway 4, or the rolling contact surface of each of the rolling elements 5, which is undesirable. Therefore, conventionally, the thickness dimension of the part of the ceramic sprayed layer that covers the continuous folded over portions 9 is also reduced by grinding. Grinding the surface parts of the continuous folded over portions 9 causes an increase in the cost, for no gain.
Furthermore, conventionally, an invention disclosed in Patent Documents 4 and 5 is known as a technique that aims at preventing a ceramic insulating coating from being damaged. The invention disclosed in Patent Document 4 improves the toughness of the insulating coating by impregnating a ceramic insulating coating with a synthetic resin. Moreover, the invention disclosed in Patent Document 5 prevents the ceramic sprayed layer from breaking off, by covering a ceramic insulating coating with a metal layer. However, in the cases of the inventions disclosed in Patent Documents 4 and 5, manufacture is difficult, so neither of them can achieve low cost.
On the other hand, Patent Document 6 discloses an invention related to an electrolytic erosion preventing insulated rolling bearing having a gray alumina insulating coating containing titanium oxide (TiO2) at 1 percent by weight or less. As disclosed in Patent Document 6, in the case where only performance maintenance (maintenance of insulation resistance value) of an insulating coating is considered, white alumina, being pure alumina that does not contain titanium oxide, is superior to gray alumina containing titanium oxide. However, as is disclosed in Patent Document 6, in the case of white alumina, the yield of the material (alumina grain) at the time of sprayed layer formation is poor, increasing the cost. Therefore, in the case of the invention disclosed in Patent Document 6, insulation performance is maintained while controlling the cost increase by using gray alumina having the above-described composition.
However, according to an experiment performed by the inventor of the present invention, it is found that gray alumina having the composition disclosed in Patent Document 6 does not always maintain sufficient insulation performance. On the other hand, in the case where white alumina is used, it is found that, if the grain diameter of alumina sprayed is controlled appropriately, although the amount of cost increase is limited, irregular coloring occurs on the surface, resulting in a poor external appearance of the product. That is, the alumina sprayed layer has minute voids inside it in its normal condition, and in the case where moisture enters the voids, the insulation performance deteriorates. Therefore, after forming the sprayed alumina layer, as is disclosed in Patent Document 7, it is necessary to perform sealing whereby the above-described voids are covered with synthetic resin, so that the moisture is prevented from entering the voids. In the case where the sprayed layer is formed using white alumina, irregular coloring occurs on the surface of the sprayed layer accompanying the sealing with synthetic resin. Such irregular coloring is not a problem in terms of insulation performance, but since it degrades the external appearance of the product, it is not desirable.
On the other hand, as shown in FIGS. 35 and 36, there is a construction in which seal rings 10, being sealing devices, or shield plates 11, are provided in a rolling bearing. In the case of the construction shown in FIG. 35, the seal rings 10 are formed by reinforcing an elastic material 13 such as rubber or the like with metal cores 12. The outer peripheral edges of the two seal rings 10 are fitted in fitting grooves 14 formed in the inner peripheral surfaces of the two ends of the outer ring 3a, and the inner peripheral edges of the two seal rings 10 are pressed into sliding contact with parts of the outer peripheral surfaces of the two ends of the inner ring 1a around the entire circumference. Furthermore, in the case of the construction as shown in FIG. 36, the shield plates 11 are formed by a metal plate formed approximately circularly, and the outer peripheral edges are fitted into the fitting grooves 14 formed in the inner peripheral surfaces of the two ends of the outer ring 3a, and the inner peripheral edges are close to the outer peripheral surfaces of the two ends of the inner ring 1a. In the cases of the constructions as shown in FIGS. 35 and 36, by providing the seal rings 10 or the shield plates 11, the space between the outer ring 3a and the inner ring 1a, where each of the rolling elements 5 are installed, is isolated from the external environment.
Heretofore, in the case of forming the aforementioned insulating coating 6 as shown in FIG. 34 in the abovementioned constructions as shown in FIGS. 35 and 36, the fitting grooves 14 are not covered with the insulating coating 6 as shown in FIG. 37. Therefore, as shown in FIG. 38, in a state in which the outer ring 3a is installed in a metal housing 15 that forms a rotation support portion such that part of the housing 15 and the two fitting grooves 14 are adjacent, the distance between the surfaces of the fitting grooves 14, which are not covered with the insulating coating 6, and the surface of part of the housing 15, becomes small. In this case, there is a possibility that discharge phenomena occur between the surface of part of the housing 15 and the two fitting grooves 14. Especially, in the case where the potential difference is great (for example, 1500V or more), such discharge phenomena are likely to occur. If the discharge phenomena occur, current flows into the rolling bearing, and there is a possibility that the aforementioned electrolytic erosion will occur.
Furthermore, in recent years, in order to improve sealing, carbon black or silica has been added to the elastic material 13 of the seal rings 10, which are fitted into the two fitting grooves 14. For example, carbon black is added to acrylonitrile-butadiene rubber or the like as the elastic material 13 to improve the friction property, wear resistance, and thermal resistance. However, since carbon black is conductive, in the case where the fitting grooves 14 are not covered by the insulating coating 6 as described above, it is easy for current to flow through the two seal rings 10 from the two fitting grooves 14.
Moreover, in the case where the metal shield plates 11 are fitted into the two fitting grooves 14 also, since the inner peripheral edges of the two shield plates 11 are close to the outer peripheral surfaces of the two ends of the inner ring 1a, it can be expected that it is easy for current to flow through the two shield plates 11. In this manner, in the case where current flows through the seal rings 10 or the shield plates 11, there is a possibility that electrolytic erosion occurs between the inner peripheral edges of the seal rings 10 or the inner peripheral edges of the shield plates 11, and the outer peripheral surfaces of the two ends of the inner ring 1a. In the case where electrolytic erosion occurs in such places, the seal deteriorates, leading to a shortened life of the rolling bearing.
Furthermore, in the case where the seal rings 10 or shield plates 11, as described above, are conductive, there is a possibility that discharge phenomena occur between the housing 15 and the seal rings 10 or the shield plates 11. In the case where such discharge phenomena occur, current flows not only between the seal rings 10 or the shield plates 11 and the inner ring 1a, but also through the fitting grooves 14 to the outer ring 3a side. Therefore, there is a possibility that the aforementioned electrolytic erosion occurs even among the component members of the outer ring 3a, the rolling elements 5 and the inner ring 1a. 
Moreover, in order to prevent electrolytic erosion as described above, heretofore, a technique is known in which an insulating coating made from synthetic resin with excellent insulating characteristics, is formed on the outer peripheral surface of the outer ring or the inner peripheral surface of the inner ring, which are surfaces that fit with the housing or the shaft. As such a synthetic resin with excellent insulating characteristics, polybutylene terephthalate (PBT), polyamide 66 (PA66), polyamide 6 (PA6) and the like are recommended for example. However, among them PA66 and PA6 have high water absorptivity, and their dimensions can change easily by absorbing moisture from the air. Therefore, they are not desirable as materials for coating the fitting surface of a bearing device, which must be accurate. Moreover, the above-described PBT is sometimes not sufficiently thermal resistant or strength, so it is inevitably not desirable as a material for coating the interface surface.
In response, in Patent Document 11 for example, a technique is disclosed wherein polyphenylene sulfide (PPS) containing glass fiber is used as a material for forming an insulating coating. That is, as shown in FIG. 39, an insulating coating 17 made from PPS containing glass fiber is formed on the outer peripheral surface and the two end faces of the metal outer ring 3b and the inner peripheral surface and the two end faces of the metal inner ring 1b, which are constituents of a rolling bearing 16. The rolling bearing 16, which is shown in the figure, is a deep groove ball bearing, and therefore there is a plurality of metal balls 18 provided between the outer ring raceway 4 formed in the inner peripheral surface of the outer ring 3b, and the inner ring raceway 2 formed in the outer peripheral surface of the inner ring 1b. In the rolling bearing 16 configured in this manner, the outer ring 3b is fitted inside the metal housing 15a via the insulating coating 17, and the inner ring 1b is fitted outside the metal shaft, which is not shown in the figure, similarly via the insulating coating 17. As a result, it is possible to prevent current from flowing into the rolling bearing 16, and thus prevent electrolytic erosion.
Furthermore, by using PPS that is reinforced by glass fiber as described above as the insulating coating 17, it is possible to solve problems such as dimensional change due to moisture absorption, and insufficiency of thermal resistance and strength as mentioned previously. However, synthetic resin material as described above has a greater coefficient of linear expansion than that of metal such as bearing steel, which is the material for the bearing ring of the rolling bearing, the housing, the shaft, and the like. Accordingly, it can be deformed easily due to the heat generated in the rotation support portion.
In response, Patent Documents 6, 12 and 13 disclose a technique wherein a ceramic insulating coating, whose coefficient of linear expansion is small, is formed on the outer peripheral surface of the outer ring and the two end faces. In the case of the construction disclosed in Patent Document 12, as shown in FIG. 40, a ceramic insulating coating 17a is formed on the outer peripheral surface and the two end faces of an outer metal ring 3c, and the insulating coating 17a is covered with a metal layer 19. A rolling bearing 16a, which is shown in the figure, is a cylindrical roller bearing. Therefore, flange parts 20 are formed on the inner peripheral surfaces of the two ends of the outer ring 3c, and a cylindrical outer ring raceway 4a is formed on the inner peripheral surface of the central part of the outer ring 3c, which is located between the two flange parts 20. Moreover, a cylindrical inner ring raceway 2a is formed on the outer peripheral surface of the central part of the inner metal ring 1c. A plurality of cylindrical metal rollers 21 is provided between the outer ring raceway 4a and the inner ring raceway 2a. 
In the case of the rolling bearing 16a configured as above, since the insulating coating 17a is made from a ceramic whose coefficient of linear expansion is small, deformation due to heat can be suppressed. Furthermore, since the insulating coating 17a is covered with the metal layer 19, when the outer ring 3c is fitted inside the mating component such as the housing or the like by an interference fit, the insulating coating 17a can be prevented from peeling off. However, in the case of the construction disclosed in Patent Document 12 having such a configuration, the manufacturing cost increases proportionate to forming the metal layer 19. Moreover, Patent Document 13 discloses a construction in which a first metal layer is provided on the outer peripheral surface of the outer ring and the two end faces, an insulating coating is provided on the first metal layer, and furthermore a second metal layer is provided on the insulating coating. In the case of the construction disclosed in Patent Document 13, it is also inevitable that the cost will be increased.
Incidentally, an electric motor or an electrical generator, into which a rolling bearing provided with an insulating coating as described above is incorporated, is generally inverter controlled. Furthermore, in recent years, in order to reduce noise at the time of switching, there has been a tendency to increase the carrier frequency of the inverter. Consequently, the current flowing into the rolling bearing has become high frequency. Accordingly, the insulating coating is required to have high impedance (a high insulation resistance value). The impedance becomes smaller as the capacitance (C) increases (or becomes greater as C decreases) as is evident from the following equation.
                                        Z                          =                  1                                                    1                                  R                  2                                            +                                                (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    fC                                    )                                2                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
|Z|: impedance (Ω)
R: resistance (Ω)
f: frequency (Hz)
C capacitance (F)
Accordingly, in order to increase the impedance |Z|, it is necessary to reduce the capacitance C. This capacitance C is proportional to the area (A) as is evident from the following equation.
                    C        =                              ɛ            0                    ⁢                      ɛ            r                    ⁢                      A            S                                              [                  Equation          ⁢                                          ⁢          2                ]            
∈o: dielectric constant of a vacuum (8. 854×10−12 F/m)
∈r: relative dielectric constant
A: area (m2)
S: distance (m)
Accordingly, in the case where the thickness (distance S) of the insulating coating is constant, the smaller the area A, the smaller the capacitance. Accordingly, in the case where an insulating coating with the same thickness is applied to rolling bearings with different bearing sizes, the capacitance of a rolling bearing with a larger bearing size, whose surface area becomes larger, is larger than that of a rolling bearing with a smaller bearing size. Therefore, in order to increase the impedance of an insulating coating applied to a rolling bearing with a larger bearing size, it is necessary to increase the thickness of the insulating coating. However, if the thickness of the film increases, the cost of the material increases. Especially, in the case where the insulating coating is made of a ceramic sprayed layer, the duration of the spraying operation increases, which also incurs an increase in the cost.
Patent Document 6 discloses a construction in which a ceramic insulating coating consists of only one layer formed directly on the material surface. However, in the case where the construction disclosed in Patent Document 6 is used in a large sized rolling bearing, there is a possibility that sufficient insulation performance cannot always be obtained. The reason is that the case of the construction disclosed in Patent Document 6 is targeted at a rolling bearing whose outer diameter is approximately 120 to 170 mm, and a large-sized rolling bearing whose outer diameter is 200 mm or more is not taken into consideration. As mentioned above, in the case where an insulating coating is formed on the outer ring of a large-sized rolling bearing, the film thickness must be increased as the surface area is enlarged. However, if the film thickness is increased by greater than or equal to a certain value, good insulation performance proportionate to the increase in the cost cannot always be expected. Accordingly, in the case of considering the material cost and the spraying operation duration, depending on the quality of the insulating coating material, it is sometimes difficult to obtain an insulating coating with sufficient insulation performance.
Furthermore, in the case where a ceramic insulating coating is formed on a rolling bearing, in many cases it is formed on the outer ring side. The reason is that in the case of forming a ceramic insulating coating by spraying, for the outer ring whose outer peripheral surface and two end faces are the surfaces to be sprayed, spray nozzles can be placed on the outside of the component, improving manufacturability. Moreover, in the case where the rolling bearing is incorporated in a rotation support portion, in many cases the inner ring and the shaft are fitted together by an interference fit, and the outer ring and the housing are fitted together by a clearance fit. However, in the case where the brittle ceramic insulating coating is formed on the inner peripheral surface of the inner ring, which is an interference fit, there is a possibility that cracks or chips occur in the insulating coating. Therefore, in order to prevent such cracks or chips occurring in the insulating coating, in many cases, the ceramic insulating coating is formed on the outer ring side.
However, the sum of the surface areas of the outer peripheral surface of the outer ring and the two end faces is larger than the sum of the surface areas of the inner peripheral surface of the inner ring and the two end faces, so in the case of forming an insulating coating on the outer ring side, from the equation relating the capacitance and the area as mentioned previously, it is necessary to increase the film thickness in order to decrease the capacitance. In response, a construction can be considered in which a ceramic insulating coating is formed on the inner peripheral surface of the inner ring, the inner ring and a shaft are fitted together by a clearance fit, and disparity between the fitted surfaces is prevented by the design of the shape of the fitted surface, or using a jig or the like. However, since creeping occurs between the inner ring and the shaft, this is not realistic. Furthermore, Patent Document 6 discloses a construction in which the ratio of titanium oxide contained in a ceramic is controlled. In the case of the construction disclosed in Patent Document 6, the titanium oxide is regulated to 0.25 to 0.75 percent by weight. However, in the case where the ratio of the titanium oxide is increased this much, there is a possibility that sufficient impedance cannot be ensured.
Patent Documents 14 to 16 are documents in which a technique associated with the present invention is disclosed. Patent Documents 14 and 15 disclose a technique in which the dimensions of a bearing are stabilized even at a high temperature. Moreover, Patent Document 16 discloses a case where an inner ring is made from a ceramic, and a technique in which the interference between an inner ring and a shaft is regulated.
On the other hand, in the case of the known conventional electrolytic erosion preventing insulated rolling bearing, which is disclosed in the aforementioned Patent Documents 1 to 3, it is found by research by the present inventor that under harsh working conditions, such as the case where the insulating coating 6 (refer to FIG. 34) is thinned in order to reduce cost, there is room for improvement from the aspect of preventing electrolytic erosion. For example, in the case of the conventional construction disclosed in Patent Documents 1 and 2, a continuous portion 22 of the axial end faces of the outer ring 3 (refer to FIG. 34), being a coated bearing ring, and the inner peripheral surface of the outer ring 3, being a raceway side peripheral surface, intersect as shown in FIG. 41 in detail. That is, the continuous portion 22 is a vertex with an angle of 90 degrees. However, in the case of the conventional construction disclosed in Patent Document 3, as shown in FIG. 42, a partial cone indented chamfer 23, which is inclined at 45 degrees with respect to the central axis of the outer ring 3, is formed at the continuous portion of the inner peripheral surface and the axial end face of the outer ring 3. The continuous portion 22a between the outer peripheral rim of the chamfer 23 and the axial end face is a vertex with an angle of 135 degrees.
As described above, either in the construction shown in FIG. 41, or in the construction shown in FIG. 42, the continuous portions 22 and 22a are vertices. In the case where current flows between the outer ring 3 and a housing 15b (refer to FIG. 42), the current tends to concentrate at the continuous portions 22 and 22a. As a result, it is found by research including a range of experiments by the present inventor that there is a case in which, even in the case of a rotation support portion of a rotating shaft of an electric motor for general purposes or rail car, or a rotating shaft of an electrical generator, sparks occur between the continuous portions 22 and 22a, and the housing 15b. That is, it is found (it was not known prior) in the process of the research for understanding the influence of thinning the insulating coating by the present inventor, that sparks occur between the continuous portions 22 and 22a, and the housing 15b. 
Such sparks can be prevented from occurring to a certain extent by increasing the thickness of the insulating coating 6 coating the continuous portions 22 and 22a. For example, in the case where an electrolytic erosion preventing insulated rolling bearing is used for a rotation support portion of the abovementioned electric motor or electrical generator, if the thickness exceeds 0.3 mm, sparks can be prevented to a considerable degree. Furthermore, if the thickness exceeds 0.5 mm, most of the sparks can be prevented. However, in order to increase the thickness, it is necessary to perform operations of coating the insulating coating 6 a plurality of times (multi-layer coating), causing an increase in the cost of manufacturing the electrolytic erosion preventing insulated rolling bearing, which is undesirable. Moreover, sparks can be prevented from occurring completely if the housing 15b or an outer ring spacer, which the continuous portions 22 and 22a face, is made from insulating material. However, from the aspects of strength or cost, in most of cases, it is difficult to make the housing 15b or the outer ring spacer from insulating material.
Moreover, it is common practice that on part of the surface of an inner ring 1 or an outer ring 3 constituting a rolling bearing, characters or reference symbols indicating the performance, the product number, the lot number, or the like, of the rolling bearing in which the inner ring 1 or outer ring 3 is incorporated, are inscribed. In the case of a typical rolling bearing, as shown in FIG. 43, conventionally, the characters or reference symbols (part indicated “****001” in the figure) are inscribed using a laser marker, an air marker (minute drill-like cutting tool, which is rotated by compressed air) or stamp, as disclosed in Patent Document 17 for example. Furthermore, Patent Document 18 discloses that the assembly direction of an outer ring is indicated on part of the outer ring using an inscribed line, or the like.
Even if a construction in which characters or reference symbols are inscribed on the axial end faces of the inner ring 1 or the outer ring 3 as shown in FIG. 43 is used in the electrolytic erosion preventing insulated rolling bearing as shown in FIG. 34, there is a possibility that the characters or symbols are covered, preventing them from being discriminated. Moreover, even if they can be discriminated, an edge (vertex) occurring due to marking is likely to be a start point of discharge, and in the case where discharge occurs, damage due to electrolytic erosion is produced in the rolling bearing.    [Patent Document 1] Japanese Patent Application Publication No. Hei 01-182621    [Patent Document 2] Japanese Patent Application Publication No. Hei 05-52223    [Patent Document 3] Japanese Patent Application Publication No. Hei 05-312216    [Patent Document 4] Japanese Utility Model Application Publication No. S 60-85626    [Patent Document 5] Japanese Utility Model Publication No. Hei 06-2030    [Patent Document 6] Japanese Patent Application Publication No. 2005-133876    [Patent Document 7] Japanese Patent application Publication No. 2003-183806    [Patent Document 8] Japanese Patent Publication No. 2571594    [Patent Document 9] Japanese Patent Publication No. 3009516    [Patent Document 10] Japanese Patent Application Publication No. Hei 07-279972    [Patent Document 11] Japanese Patent Publication No. 2779251    [Patent Document 12] Japanese Patent Application Publication No. 2002-48145    [Patent Document 13] Japanese Patent Application Publication No. 2002-181054    [Patent Document 14] Japanese Patent Publication No. 3475497    [Patent Document 15] Japanese Patent Publication No. 2624337    [Patent Document 16] Japanese Patent Publication No. 2617300    [Patent Document 17] Japanese Patent Application Publication No. 2005-42895 (Paragraph 0003)    [Patent Document 18] Japanese Patent Application Publication No. 2005-214348 (Paragraph 0040)